New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary analysis of geochemical and petrographic data from 40 Cerros del Rio lavas suggests the following: 1) spatial distribution of lava types may support the existence of a tholeiitic shield volcano to the northwest of the Cerros field; 2) quartz phenocrysts found in the lavas are most probably xenocrystic in nature, rather than high pressure phases: and 3) crustal contamination occurred during the evolution of cerros del Rio magma.

Cerros del Rio samples were collected on the east side of the Rio Grande River from Ortiz mountain and Sagebrush flats, and on the west side of the river from various locations including Frijoles and Alamo canyons. The dominant lava types found on the east side are classified as hawaiites and andesites with interfingering tholeiites. On the west side of the river, dominant lava types are classified as tholeiites and andesites with minor amounts of interfingering hawaiites. The range of chemical variation among all the lavas sampled is large and may confirm the suggestion that a shield volcano which tapped a different source than that of the Cerros is buried under Bandelier Tuff to the northwest.

Petrographically, the occurrence of disequilibrium textures in plagioclase megacrysts, in addition to the occurrence of quartz xenocrysts in all rock types is consistent with contamination by a felsic component. Also, in some andesites, olivine is rimmed by opx reaction halos, suggesting that the magma was subjected to mixing with a relatively silica-enriched component. Some quartz found in the lavas has a strained
metamorphic texture and small, oriented microlites of rutile indicating that the ultimate source for the quartz was a metamorphic rock.

The existence of Nb and Ta troughs in chondrite-normalized incompatable alement diagrams for the Cerros del Rio lavas is consistent with contamination by crustal material. Incompatable element ratios (Th/Ta and La/Yb) increase with increasing Fe/(Fe+Mg) suggesting the importance of AFC processes in the differentiation of these lavas.